Q1: What is the modem hang up command?

ANS:B - ATH

H0 Force modem on-hook (hang-up)
H1 Force modem off-hook (to answer or dial).
Modem AT Commands
Moving between command and on-line states

When the modem is turned on or after reset, the modem always goes to command state. It enters on-line state after successfully making a connection with a remote modem, either when answering or originating a call.

While the modem is in on-line state, you can take control of it again (go back to command state). When you return to command state from on-line state, the modem goes to local command state, which allows you to maintain the connection and enter commands.

To go to local command state, type the escape sequence +++. Do not type any other commands or make entries before or after the escape sequence for a period equal to the guard time set in register S12 (normally one second). The modem will display OK.

To go back to on-line state, type ATO.

To hang up, type ATH.
Hn Hook (hang up)
When the modem is on-line, you must first enter the +++ escape sequence to take the modem back to command state before issuing the hang up command, ATH.

0 - Forces the modem on-hook. Hangs up the modems connection to the telephone line and place modem in command state.

1 - Forces the modem off-hook. After going off hook, operate auxilary relay.
AT commands are used to control MODEMs. AT is the abbreviation for Attention. These commands come from Hayes commands that were used by the Hayes smart modems. The Hayes commands started with AT to indicate the attention from the MODEM.